1. Field of the Invention
Embodiments of the invention generally relate to apparatus and methods of measuring conditions in a well-bore.
2. Description of the Related Art
Distributed Temperature Sensing (DTS) enables monitoring temperature along the length of a well. A DTS system utilizes an optical waveguide, such as an optical fiber, as a temperature sensor. In a typical DTS system, a laser or other light source at the surface of the well transmits a pulse of light into a fiber optic cable installed along the length of the well. Due to interactions with molecular vibrations within glass of the fiber, a portion of the light is scattered back towards the surface. A processor at the surface analyzes the light as it is sent back. The processor then determines the temperature at various depths within the well, based on the reflected light.
A problem with DTS systems is that the signal reflected back to the processor is weak and can be difficult to read. This problem is especially true for long waveguides in deep wells. Therefore, the weak signal makes it difficult to accurately determine the temperature in deep well-bores.
Utilizing an Array Temperature Sensing system (ATS) overcomes the weak signal of the DTS system. In the ATS system, several Bragg gratings are placed in a waveguide, such as a fiber. The gratings can be at any desired location along the waveguide. Advantageously, a reflected signal from the grating is greater than that of the DTS system.
A major challenge to the use of an ATS system involves the packaging of the Bragg gratings such that they are responsive to the temperature of their surroundings but are free from, or insensitive to, strain changes over their lifetime. The effects of these strain changes are generally indistinguishable from those of changes in temperature and cause errors in the temperature measurement.
DTS measurement is also sensitive to changes in the loss and refractive index of the optical fiber being interrogated, while the gratings of the ATS system are sensitive to changes in the refractive index and physical dimension of the fiber. In many cases, such changes happen over the lifetime of the system. For example, production fluids and gases, particularly hydrogen, in a well-bore can cause significant increases in the fiber loss and refractive index of glass optical fibers. The ingress of production fluids, e.g., water, can cause swelling of the glass optical fibers which changes the measured wavelength and hence measured temperature of the Bragg gratings in the ATS system.
Therefore, there exists a need for an improved ATS system that reflects a stronger signal than a DTS system and eliminates or at least reduces adverse effects of changes in strain over the system lifetime. A further need exists for methods and assemblies to provide the ATS system that is protected from the ingress of fluids and gases.